This invention relates to a photographic element for use in a process for forming an image through heat development, and more particularly, to such a photographic element having an exothermic layer.
Heat developable photosensitive materials and heat development process are well known in the art and described in the literature, inter alia, "Fundamentals of Photographic Engineering", Corona Publishing K.K., Tokyo, Japan (1979), pages 553-555; "Image Information", April 1978, page 40; Nebletts Handbook of Photography and Reprography, 7th ed., Van Nostrand Reinhold Company, pages 32-33; U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020, and 3,457,075; British Pat. Nos. 1,131,108 and 1,167,777; and Research Disclosure, June 1978, pages 9-15 (RD-17029).
A process for transferring a mobile dye imagewise formed by heat development to an image-receiving layer by heating and image-receiving materials used therefor are described in Japanese Patent Application Kokai Nos. 58-58543, 58-79247, and 59-168439, inter alia.
Heating of these heat-developable photosensitive materials and image-receiving materials (both generally referred to as photographic materials, hereinafter) may be carried out by a number of methods including contacting of photographic materials with a heat block having a great capacity; direct heating of photographic materials by laser and infrared irradiation, ultrasonic heating, high frequency heating or the like; and passing of photographic materials through heated gas. These methods are, however, not successful in achieving satisfactory results because of their shortcomings. For instance, in the use of a heat block, is time and power consuming to accomplish a uniform temperature distribution throughout the heat block, and insufficient contact prevents smooth uniform heat transfer to the photographic material. The use of radiation such as by a laser beam is disadvantageous in that a large sized apparatus is required or a compact system is difficult to incorporate. The use of heated gas is time consuming because the gas has essentially a low heat capacity.
To overcome these shortcomings, the use of an exothermic electroconductive layer in combination with a heat developable photosensitive layer was proposed as disclosed in, for example, U.S. Pat. No. 206,368 and Japanese Patent Application Kokai No. 48-66442.
Also, a variety of positive electroconductive layers (which herein designate electroconductive layers whose electric resistance increases with a temperature rise) intended for plane heaters were developed as disclosed in, for example, Japanese Patent Application Kokai Nos. 49-82734, 49-82735, 51-13991, 51-39742, 51-39743, and 52-87694.
These methods aim to prevent overheating by taking advantage of the nature of these materials, i.e., their electric resistance increases with a temperature rise, and employ these materials as plane heaters and in snow melting systems. It is possible to apply these positive electroconductive layer to photographic materials and such an application is described, for example, in Japanese Patent Application No. 58-229377.
Any desired choice of design may be made on an exothermic electroconductive layer so as to meet the intended application whether it is of the positive type as mentioned above, of the negative type wherein the electric resistance of an electroconductive layer decreases with a temperature rise, or of the neutral type wherein resistance does not depend on temperature. With respect to a binder used in such a layer, a choice may be made between hydrophilic binders and hydrophobic polymeric binders combined with organic solvents, depending on the desired properties of the electroconductive layer.
When these exothermic electroconductive layers are used in combination with heat-developable photosensitive layers as photographic materials, the electroconductive layers must be imparted with properties other than electroconductive properties. In the case of a sheet- or roll-shaped sample, interfacial adhesion must be prevented between a photosensitive layer and an electroconductive layer when these layers overlap one another, marring on the electroconductive layer surface must be prevented during mechanical transfer of the sample, and an improvement is required in overcoming variation of resistance value when the sample and developing equipment are held under different ambient conditions including relative humidity and temperature.
Such considerations are essential to accomplish stable heat development and transfer of a mobile dye by heating and not fully met by the conventional photographic material. There is the continuing need for a further improvement in this respect.